Constraints Editor
Specifying Registers to be Placed in Input/Output Blocks (IOBs)
You can use the Registers to be Placed in IOBs (IOB) dialog box to specify which flip-flops and latches will be moved into IOBs when the device is mapped. Only the first flip-flop or latch encountered in the path from a pad can be moved into the IOB. Placing a flip-flop or latch into an IOB moves it closer to the pad, so the pad to setup time decreases but the delay from the IOB flip-flop or latch to the next synchronous element increases.
To Specify Registers to be Placed in IOBs
  1.  Open the Registers to be Placed in IOBs (IOB) dialog box.
    •  In the Timing Constraints panel, select Miscellaneous > Registers to be Placed in IOBs and in the Workspace, right click the constraint to edit and select Edit Constraint to edit a constraint, or right click the bottom row of the table and select Create Constraint to create a new constraint.
    •  Cross probe from another program.
  2.  To limit the number of selections in the Available registers for IOB packing list, enter a filter string in the Filter box and click Find. For example, to list only registers containing the string "b/Q," type *b/Q* in the Filter box. The filter is not case sensitive.
  3.  In the Available registers for IOB packing list, select the registers to add to IOBs.
  4.  Click the Add button. Image
    Selected registers will be moved to the Registers for IOB packing list.
  5.  Click Create to create the specified constraint without closing the dialog box, or OK to create the constraint and close the dialog box.
Specifying registers to be placed in IOBs creates an IOB constraint for each selected flip-flop or latch. The new constraint appears in the current Workspace table.
For more information about this constraint, see the Constraints Guide in the online document collection.

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